Liquid crystal display devices have been used for clocks, calculators, household electric appliances, measuring instruments, panels for automobiles, word processors, electronic organizers, printers, computers, televisions, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, an STN (super twisted nematic) mode, a DS (dynamic scattering) mode, a GH (guest-host) mode, an IPS (in-plane switching) mode, an OCB (optically compensated birefringence) mode, an ECB (electrically controlled birefringence) mode, a VA (vertical alignment) mode, a CSH (color super-homeotropic) mode, and FLC (ferroelectric liquid crystal). The driving method has been changed from conventional static driving to multiplex driving, and passive matrix driving and, recently, active matrix (AM) driving performed using, for example, TFTs (thin film transistors) and TFDs (thin film diodes) have become the predominantly used driving method.
Referring to FIG. 1, a typical liquid crystal color display device includes two substrates (1) each having an alignment film (4), a transparent electrode layer (3a) serving as a common electrode and a color filter layer (2) which are disposed between one of the substrates and the alignment film of the one substrate, and a pixel electrode layer (3b) between the other substrate and the alignment film of that other substrate. The substrates are arranged so that the alignment films face each other and a liquid crystal layer (5) is sandwiched between the alignment films.
The color filter layer is constituted by a color filter that includes a black matrix, a red colored layer (R), a green colored layer (G), a blue colored layer (B), and, if needed, a yellow colored layer (Y).
Liquid crystal materials constituting such liquid crystal layers have been subjected to high levels of impurity control since impurities remaining in the materials significantly affect electrical properties of display devices. Regarding the materials that form alignment films, it has been known that the alignment films come into direct contact with the liquid crystal layer and impurities remaining in alignment films migrate to the liquid crystal layer, so that the impurities affect electrical properties of the liquid crystal layer. Studies are now being conducted in order to determine the properties of liquid crystal display devices affected by the impurities in the alignment film materials.
Materials, such as organic pigments, used in the color filter layer are also presumed to affect the liquid crystal layer due to impurities contained in the materials as with the case of the alignment film materials. However, since an alignment film and a transparent electrode are interposed between the color filter layer and the liquid crystal layer, the direct effects on the liquid crystal layer have been considered to be significantly low compared to those of the alignment film materials. However, alignment films are usually as thin as 0.1 μm or less in thickness. Transparent electrodes that serve as color-filter-layer-side common electrodes are thick so as to enhance the electrical conductivity; however, the thickness thereof is usually only as large as 0.5 μm or less. Accordingly, the color filter layer and the liquid crystal layer are not completely separated from each other. There is a possibility that impurities contained in the color filter layer may migrate through the alignment film and the transparent electrode and cause a decrease in the voltage holding ratio (VHR) and an increase in the ion density (ID) in the liquid crystal layer, thereby leading to display defects such as white streaks, variations in alignment, and image sticking.
Studies have been conducted to find a way to resolve display defects caused by impurities contained in pigments in color filters: a method of controlling release of impurities into liquid crystals by using a pigment in which the content of extracts obtained with ethyl formate is limited to a particular value or less (PTL 1) and a method of controlling release of impurities into liquid crystals by specifying the pigment in the blue colored layer (PTL 2). However, these methods do not differ much from simply decreasing the amounts of impurities in the pigment and fail to provide sufficient improvements that resolve the display defects even under the recent progress in pigment purification technologies.
Also disclosed are a method that focuses on the relationship between organic impurities contained in the color filter and a liquid crystal composition, in which insolubility of the organic impurities in the liquid crystal layer is indicated by a hydrophobicity parameter of liquid crystal molecules contained in the liquid crystal layer and the value of this hydrophobicity parameter is controlled to a particular value or higher and a method of preparing a liquid crystal composition that contains a particular fraction or more of a liquid crystal compound having a —OCF3 group at an end of the liquid crystal molecule since there is a correlation between this hydrophobicity parameter and the —OCF3 group at an end of a liquid crystal molecule (PTL 3).
However, the essence of the invention disclosed in this literature is to suppress effects of impurities in the pigment on the liquid crystal layer and thus a direct relationship between the structure of the liquid crystal material and the properties of the coloring material itself such as dyes and pigments used in the color filter has not been investigated. This literature does not resolve the problems related to display defects of liquid crystal display devices that have become sophisticated.